1. Field of the Invention
The present invention relates to an antenna for transmitting and receiving radio waves of a megacycle (MHz) or gigacycle (GHz), and particularly to an antenna device which can be structured in a thin shape, has a broad tuning frequency band, directivity, high gain, and which can be manufactured inexpensively.
2. Prior Art Statement
FIG. 1A is a side view showing a prior art example of a planar antenna with a reflector, and FIG. 1B is the perspective view thereof.
Reference numeral 6 refers to an emission plate and reference numeral 5 refers to a reflector (see both FIG. 1A and FIG. 1B).
Reference numeral 6a is the center portion of the emission plate 6, and at this point the impedance is 0, the current value is maximum and the voltage value is 0.
The impedance changes continuously from the center portion 6a to the end portion 6b. Point 7 of the impedance of 50 xcexa9 during such change is the feeding point, and a center conductor 8a of a coaxial cable 8 is connected thereto. The outside conductor 8b of the coaxial cable 8 is connected to the reflector 5.
The aforementioned reflector 5 and emission plate 6 are supported in parallel with the connection conductor 9 at an interval measurement of L.
In this planar antenna example, the radio wave reflected at the reflector 5 is emitted in the arrow Z direction at a maximum of 3 dBd. In terms of bandwidth ratio, the areas of VSWR 2.0 or less are 3 to 5% or less.
FIG. 2A is a side view of a prior art example in which the planar antenna of FIG. 1A was improved in order to obtain broad band characteristics, and FIG. 2B is the perspective view thereof.
Reference numeral 11 refers to an inverted-F antenna element, 11a refers to the grounding point thereof, and 11b refers to the open end thereof.
The open end 11b of this inverted-F antenna element 11 forms the static coupling capacity c by facing and being distanced from the reflector 10. At this open end 11b, the impedance is infinite, the current value is 0, and the voltage value is maximum.
At the grounding point 11a, the voltage value is 0 and the current value is maximum, and these values change continuously between the open end 11b and the grounding point 11a. Point 11c having an impedance of 50 xcexa9 during such change is the feeding point, and a center conductor 8a of a coaxial cable 8 is connected thereto.
The electrical length between the end portion 6b and end portion 6c of the emission plate is a half wavelength, and the supporting body 10 supporting the center portion 6a thereof may be either a conductor or an insulator.
The bandwidth ratio of the prior art example shown in FIG. 2A and FIG. 2B is slightly lower than 10%. The gain is approximately the same as the previous example (FIG. 1A and FIG. 1B), but shows a slight increase.
The thickness measurement (measurement in the Z axis direction) of the antennae of the prior art examples illustrated in FIG. 1A, FIG. 1B, FIG. 2A, and FIG. 2B is comparatively large, and, for instance, will be roughly 20 to 30 mm when designed and manufactured for use at 2.45 GHz. When designed and manufactured for a lower frequency, the thickness will be even larger.
FIG. 3 is a two-view diagram of a publicly known patch antenna. The basic structure of this patch antenna is the same as the prior art examples depicted in FIG. 1A and FIG. 1B, and, therefore, the antenna characteristics are also approximately the same.
The patch antenna is structured from a two-layer substrate shown with reference numerals 21 and 22, a ground plate 26 is formed on one of the faces of this two-layer substrate and a circular antenna element 23 is formed on the other face thereof, respectively with a conduction pattern, and are mutually connected and conducted with a short pin 25 passing through the two-layer substrate.
And, a contact pin 27 is bonded to the feeding point of the foregoing circular antenna element 23 with solder 28 and thereby connected to the strip line 24.
This conventional example, as evident from the structure illustrated in FIG. 3, is structured to have a thickness measurement of two substrates worth of thickness.
Although it is advantageous in that the structure is simple, there is no room for any other improvement in the antenna performance.
Thus, an object of the present invention is to xe2x80x9cprovide an antenna device suitable in transmitting and receiving radio waves in megacycles or gigacycles, capable of being structured in an extremely thin shape, having a simple structure and low manufacturing cost, yielding superior antenna characteristics (particularly broad band, high gain, directivity), and capable of being structured to have dual band or triple band capability.
As described in detail later, the present invention is an improvement of the slotted bow tie antenna.
Thus, background art relating to a xe2x80x9cbow tie antennaxe2x80x9d and slotted antenna is described briefly below.
FIG. 4A is a publicly known dipole antenna. (For ease of reading, the conductive portions are shown with spots in FIG. 4A to FIG. 4E.)
The dipole antenna is of the most basic structure, and FIG. 4B shows a modification thereof which is a xe2x80x9cbow tie antenna with two triangular metal plates facing each otherxe2x80x9d. As a modification of FIG. 4B, xe2x80x9ca wire bent into a trianglexe2x80x9d may be used instead of the triangular metal plate.
Reference numeral 12 refers to a high frequency power source, and the two points (1a, 1b), (2a, 2b) connected to such high frequency power source in the drawings are feeding points.
Reference numeral 3 in FIG. 4C is a slotted version of the dipole antenna 1, and a part of the metal plate 13 has been cut out.
Similarly, as shown in FIG. 4D, if the metal plate 13 is cut out in a form of a bow tie, a slotted bow tie antenna 14 can be obtained.
For the sake of explanation, the axis xxe2x80x94x illustrated in FIG. 4D will be referred to as the longitudinal symmetrical axis. In the basic form, the longitudinal symmetrical axis xxe2x80x94x is the perpendicular bisector of two sides which are parallel within the hexagon forming the bow tie shape.
The slotted bow tie antenna 14 is drawn in more detail and schematically in FIG. 5.
Reference numeral 14a is the right side, 14b is the left side, 14c is the upper right side, 14d is the upper left side, 14e is the lower right side, and 14f is the lower left side.
The center conductor 8a of the coaxial cable 8 connected to the high frequency power source 12 is connected to the feeding point 15a, and the outside conductor 8b is connected to the feeding point 15b, respectively. However, the outside conductor 8b may be connected to an arbitrary location of the metal plate 13.
The slotted bow tie antenna of the present invention is an improvement of the publicly known slotted bow tie antenna (prior art shown in FIG. 5 for example), and, with the longitudinal symmetrical axis of the bow tie shaped slot (14) set as x, and the symmetrical axis perpendicular thereto set as y, xe2x80x9ca narrow and long parasitic element insulated electricallyxe2x80x9d is placed over and across the slot (cut out portion) in the y axis direction. This is the basic structure of the present invention.
As a result of adding the aforementioned parasitic element, the present invention is able to broaden the tuning frequency band width without hindering the advantages of conventional slotted bow tie antennae such as xe2x80x9csuper thin shape,xe2x80x9d xe2x80x9csimple structure,xe2x80x9d xe2x80x9cdirectivityxe2x80x9d, xe2x80x9clow cost,xe2x80x9d and so on.
Moreover, the performance is further improved as a result of establishing two parasitic elements and structuring an array antenna by arranging a plurality of slotted bow tie antennae with parasitic elements.